1. Field of the Invention
The present invention relates to nonvolatile storage elements that do not lose stored logical states even after the power is turned off. The present invention also relates to storage devices and signal processing circuits including the storage elements. Further, the present invention relates to methods for driving the storage elements, the storage devices, and the signal processing circuits. Furthermore, the present invention relates to electronic devices including the signal processing circuits.
2. Description of the Related Art
Many central processing units (CPUs) adopt architecture called a stored program system. In the stored program system, instructions to be processed by the CPU and data necessary for processing are stored in memories (storage devices) such as a main memory and a hard disk. The CPU performs processing by sequentially reading the instructions and the data from the memories.
In the architecture, the operating speed of the CPU depends strongly on the speed of access from the CPU to the memory. In other words, in order to improve the operating speed of the CPU, it is necessary to improve the operating speed of the memory. Further, since the memory needs to store instructions to be processed by the CPU and data necessary for processing, the memory needs to have large capacity. However, a high-speed large-capacity memory is very expensive, so that such a memory cannot be easily employed.
Thus, a system is devised in which a small-capacity high-speed memory (hereinafter referred to as a cache memory), a large-capacity low-speed memory (hereinafter referred to as a main storage device or a main memory), and a larger-capacity lower-speed memory are used in combination.
Access (reading or writing) from a CPU to a main memory is controlled by, for example, a semiconductor device called a northbridge. Access to a memory that has larger capacity and lower speed than the main memory (e.g., a hard disk) is controlled by, for example, a semiconductor device called a southbridge.
A system using a CPU includes memories such as a cache memory, a main memory, and a hard disk, and a peripheral control device for controlling access (reading or writing) to these memories. Note that a peripheral device such as a PCI device, a network device, or an audio device might be included in the system using a CPU. Here, a semiconductor device for controlling these memories and the peripheral device is referred to as a peripheral control device. The northbridge and the southbridge are peripheral control devices. Further, memories such as a main memory and a hard disk also correspond to peripheral devices unless otherwise specified.
Note that the system using a CPU might be a system where the northbridge and the southbridge are merged into one peripheral control device, a system using a CPU in which peripheral control devices are merged, or the like. In the following description, a system using a CPU is referred to as a PC system. Note that a CPU or a system using a CPU is also referred to as a signal processing circuit.
Reference 1 discloses a structure where a volatile memory such as an SRAM and a nonvolatile memory are used in combination as a cache memory.